1. Field of the Invention
The present invention relates to an optical system using an optical material having extraordinary partial dispersion, and more particularly to an optical system suitable for a silver halide film camera, a digital still camera, a video camera, or the like.
2. Related Background Art
In general, chromatic aberrations such as longitudinal chromatic aberration and chromatic aberration of magnification, of an optical system deteriorate as the entire length of the optical system shortens, so that an optical performance thereof is likely to reduce. In particular, in the case of a telephoto type optical system, the chromatic aberrations increase as a focal length lengthens. Therefore, the chromatic aberrations significantly deteriorate with shortening of the entire length.
Achromatization using an extraordinary partial dispersion material and achromatization using a diffraction grating have been generally known as methods of reducing such chromatic aberrations.
In the telephoto type optical system, the chromatic aberrations are generally reduced by a front lens unit, in which a passing position of a paraxial on-axis ray and a passing position of a pupil paraxial ray from the optical axis become relatively high, including a positive lens made of a low dispersion material having extraordinary partial dispersion such as fluorite and a negative lens made of a high dispersion material. Such various telephoto lenses have been proposed (Japanese Patent Publication No. S60-049883 (corresponding to U.S. Pat. No. 4,241,983 B), Japanese Patent Publication No. S60-055805 (corresponding to U.S. Pat. No. 4,348,084 B), and Japanese Patent Application Laid-Open No. H11-119092 (corresponding to U.S. Pat. No. 6,115,188 B)).
A telephoto lens in which the chromatic aberrations are corrected using not an extraordinary partial dispersion material but a diffraction optical element has been also proposed. A telephoto lens having an F number of about F2.8 in which the chromatic aberrations are relatively preferably corrected by a combination of a diffraction optical element and a refractive optical element has been disclosed in Japanese Patent Application Laid-Open No. H06-324262 (corresponding to U.S. Pat. No. 5,790,321 B) and Japanese Patent Application Laid-Open No. H06-331887 (corresponding to U.S. Pat. No. 5,629,799 B).
With respect to features of the diffraction optical element, an absolute value of a numerical value corresponding to an Abbe number is a small value of 3.45. The chromatic aberrations can be significantly changed without substantially causing an influence on spherical aberration, coma, astigmatism, and the like by slightly changing optical power (the reciprocal of a focal length) due to diffraction. Because of diffraction light, the optical power linearly changes in accordance with a change in frequency of incident light, so that a wavelength characteristic of a chromatic aberration coefficient becomes perfectly linear. Therefore, when the entire length is shortened, the spherical aberration, the coma, and the astigmatism may be particularly corrected in aberration correction. Since the chromatic aberrations are corrected by the diffraction optical element, glass materials constituting lenses and optical powers may be suitably designed so as to obtain the linearity of the wavelength characteristic of the chromatic aberration coefficient without concern for an absolute amount of the chromatic aberrations deteriorated by shortening of the entire length. As a result, the telephoto lens whose entire length is shortened can be obtained.
A liquid material whose characteristic includes relatively high dispersion and relatively extraordinary partial dispersion has been known as a material having a chromatic aberration correction function similar to that of the diffraction optical element, and an achromatization optical system using the liquid material has been proposed (U.S. Pat. No. 4,913,535 B).
According to the telephoto lenses using fluorite or the like as described in Japanese Patent Publication No. S60-049883 (corresponding to U.S. Pat. No. 4,241,983 B), Japanese Patent Publication No. S60-055805 (corresponding to U.S. Pat. No. 4,348,084 B), and Japanese Patent Application Laid-Open No. H11-119092 (corresponding to U.S. Pat. No. 6,115,188 B), when the entire optical length is set to a relatively long length, it is possible to relatively preferably correct the chromatic aberrations. However, it is hard to correct the deterioration of chromatic aberrations due to shortening of the entire length. This is because such a method is used to merely reduce the chromatic aberrations caused in a front lens itself (lens closest to a subject) using the low dispersion and extraordinary partial dispersion of a material such as fluorite. When the chromatic aberrations deteriorated due to shortening the entire length are intended to be corrected by, for example, a lens made of low dispersion glass having a large Abbe number, such as fluorite, since it is necessary to significantly change optical power of a lens surface in order to change the chromatic aberrations, it is difficult to satisfy both the corrections of the chromatic aberrations and the corrections of various aberrations such as spherical aberration, coma, and astigmatism.
Although the diffraction optical element has a sufficient chromatic aberration correction function, there is a problem in that diffraction light of an unnecessary diffraction order other than diffraction light of an actually used design diffraction order becomes color flare light, thereby deteriorating an imaging performance. According to a so-called stack type diffraction optical element in which a plurality of blaze type diffraction gratings are stacked in an optical axis direction, energy is concentrated on the design diffraction order to significantly reduce unnecessary diffraction light. However, when a high luminance subject is photographed, there still remains a problem in that diffraction flare appears.
A method of forming an ultraviolet curable resin or the like using a metal mold has been known as a method of manufacturing the diffraction optical element. However, the diffraction efficiency of the diffraction optical element is extremely sensitive for manufacturing condition thereof. Therefore, there is also a problem in that very high mold precision and formation precision are required, thereby increasing a manufacturing cost.
Because the material disclosed in U.S. Pat. No. 4,913,535 B is liquid, a structure for sealing the material is required. Therefore, it cannot be said that manufacturing is easy. Because there is also a problem that characteristics such as a refractive index and a dispersion characteristic depends on temperature, it cannot be said that an environmental resistance is sufficient. Further it is hard to obtain a sufficient chromatic aberration correction function because an interface with air is not obtained in addition that the Abbe number is relatively large, extraordinarys partial dispersion property is relatively small.